International PCT publication WO2005/027255 is directed to a lead-acid storage battery comprising a negative electrode, which is suitable for use in a hybrid electric vehicle requiring repeated short periods of charge/discharge operation in the PSOC. The electrode is coated with a porous carbon mixture prepared by forming a paste from a binder material and a mixed powder comprising particles of a carbon material having a capacitor capacity and/or a pseudo-capacitor capacity and particles of a carbon material having electrical conductivity, which is then applied to the surface of the electrode plate and dried.
The lead-acid storage battery described in WO2005/027255 provides an increased cycle life compared with a lead-acid storage battery provided with a conventional negative plate. However, it has been found that as the charge/discharge cycle is repeated, Pb or PbSO4 is deposited on the surfaces of the particles of the carbon material having a capacitor function and the inlets of numerous internal pores of the particles become clogged with deposited Pb or PbSO4, so that the capacitor function is markedly deteriorated, and accordingly, the quick charge/discharge cycle life in the PSOC is shortened.
More particularly, with respect to the particles of the carbon material having a capacitor function, such as activated carbon or the like contained in the conventional carbon mixture coating layer, when the battery is charged to cause polarization on the lead acid negative plate for an open circuit arrangement, the material is negatively charged and adsorbs the electrical double layer protons and cations having a positive charge, and, when the battery is discharged to cause polarization on the lead acid negative plate for an open circuit arrangement, the surfaces of the particles desorb them. In addition, when the battery is further discharged to cause polarization on the lead acid negative plate (in relation to an open circuit arrangement) than the potential when not charged, the surfaces of the particles are positively charged and adsorb anions on the electrical double layer.
Thus, on the particles of the carbon material having a capacitor function, Pb ions as cations as well as protons are simultaneously adsorbed or desorbed. Therefore, the Pb ions adsorbed on the surface of activated carbon are reduced to Pb metal, and the Pb metal is deposited on the surfaces of the particles. Further, the discharging operation causes oxidation of Pb, resulting in deposition of PbSO4 on the surfaces of the particles. These particles have internal pores and hence have enormous internal surface area, but they have a seemingly smooth polyhedron or spherical external shape with a small apparent surface area. Therefore, when the deposition of Pb or PbSO4 on the outer surfaces of such particles occurs, the inlets of the internal pores are clogged with the deposited Pb or PbSO4, so that the capacitor function is markedly deteriorated.
There is a need for alternative and improved electrodes for use in lead acid battery systems, such as electrodes and batteries that improve cycle life and ameliorate some of the disadvantages of providing high rate materials in lead acid systems, particularly in systems requiring repeated short periods of charge/discharge operation in the PSOC.